Conventional magnetic scan cathode ray tubes for use to display pictorial images use a single lens means for focussing the electron beam. No correction is applied to the beam to cause the beam to be in focus over the entire display screen. Thus, as the electron beam is moved away from the screen center, it becomes slightly defocussed with the greatest amount of defocussing occurring at the areas of the screen that are furtherest from the screen center.
For display of alphanumeric information, higher resolution is required, and a bipotential lens means is generally used with a nominal focussing voltage to focus the electron beam at screen center being about 3,000 or more volts. To dynamically correct the beam focus when using a bipotential lens means when the electron beam is moved away from the screen center, a voltage of about 500 volts is added to the existing voltage on the bipotential lens means. The amplifier that is applying this voltage to the bipotential lens means must be insulated due to its high voltage operation and more power is required to dynamically correct the focus.
U.S. Pat. No. 3,603,839 employs several lens means for focussing the electron beam and a second crossover of the beam is formed to increase the beam current through a shadowmask color cathode ray tube. The voltages required to focus the second crossover are not of low value nor is the lens means that effects the second crossover used to correct for deflection defocussing.